It is known to produce gasoil and waxy raffinate from paraffinic feedstocks derived from a Fischer-Tropsch hydrocarbon synthesis process, by a combined hydrocracking/hydro-isomerisation step.
Catalysts used for hydrocracking/hydro-isomerisation of such feedstock typically are dual function catalysts comprising a hydrogenation function and an acid cracking function.
It is also known that the catalyst characteristics have an effect on the quantity and quality of the products obtained in the hydrocracking/hydro-isomerisation step. In EP 537 815 A1 for example is disclosed that a platinum on amorphous silica-alumina catalyst that is prepared from an amorphous silica-alumina starting material having a pore volume of at least 1.0 ml/g exhibits a significantly higher selectivity to middle distillates than catalysts comprising carriers prepared from starting materials having lower pore volumes.
In EP 666 894 B1 is disclosed a process for preparing a lubricating base oil from a waxy hydrocarbon feed, such as for example a synthetic wax prepared by a Fischer-Tropsch synthesis, wherein the feed is contacted in the presence of hydrogen with a catalyst comprising a hydrogenation component on an amorphous silica-alumina carrier having a macroporosity in the range of from 5 to 50 vol % and a total pore volume in the range of from 0.6 to 1.2 ml/g. Macroporosity is defined in EP 666 894 as the fraction of the total pore volume of the carrier present in pores with a diameter greater than 35 nm.
In WO 2005/005575 it is disclosed that the use of a relatively heavy Fischer-Tropsch derived feedstock in a hydrocracking/hydro-isomerisation process results in a higher yield of waxy raffinate product, i.e. the fraction boiling between 370 and 540° C., and an improved quality of the waxy raffinate product. In particular the wax content of the waxy raffinate product is reduced, resulting in improved cold flow properties and a simpler and more efficient subsequent dewaxing step.
There is still room for improvement in terms of the yield and quality of the products obtained, in particular gasoil and waxy raffinate, in a process for hydrocracking/hydro-isomerisation of Fischer-Tropsch derived feedstocks.